Many of the high solids automotive coatings presently in use are based upon polymeric systems comprised of a binder comprising either polyester-based or polyacrylic-based polyols and crosslinking agents therefor. These coatings are generally supplied as "one-pack" or "two-pack" systems.
In a typical one-pack system, all of the coating ingredients are combined into one storage-stable mixture. Upon application, the polyol component is crosslinked, generally with an aminoplast resin (such as a melamine resin) or a blocked isocyanate, usually under heat cure conditions of 120.degree. C. or above. In a typical two-pack system, the polyol component is combined with a crosslinking agent, generally an isocyanate, shortly before application, with curing being conducted at ambient and/or elevated temperatures.
For environmental reasons, it is becoming increasingly important to develop polymeric systems with low solution viscosities, which permit the formulation of high solids coatings with low application viscosities suitable for spraying. High solids coatings (generally 50 wt. % or greater solids) significantly decrease the amount of volatile organic compounds (VOC) entering the atmosphere upon drying/curing of the coating.
Further, in a typical vehicle coating refinish operation, after applying the coating to the vehicle, the resulting finish is allowed to dry before the vehicle is moved. Before any further work can be done to the finish, it must be tack free so that dust and dirt will not stick to it. It must also be hardened sufficiently to allow polishing to improve gloss and/or remove minor imperfections. Conventional high solids refinish coating compositions have long drying and curing times, and, therefore, reduce the productivity of a refinish operation since the vehicles cannot be moved and worked on quickly after application of the finish.
WO 96/02585 describes a coating composition based on a polyester polyol, especially applicable in the automotive refinish industry. It has been found that such a refinish coating composition has long drying and curing times and, therefore, has the above-mentioned disadvantage of the reduction of the productivity of a refinish operation since the vehicles cannot be moved and worked on quickly after application of the finish. AU 91685/82 (equivalent of U.S. Pat. No. 4,419,407) discloses a thermosetting coating composition comprising a polyester polyol having cyclic moieties and a polyurethane polyol. The polyurethane polyol in this publication is a reaction product of a high molecular weight polyol and a polyisocyanate compound. The Tg of the polyurethane polyol is lower than the Tg of the polyester polyol. The polyurethane polyol thus acts as a plasticizer. Accordingly, the coating compositions dry slowly and produce flexible but soft films. Similar coating compositions are described in U.S. Pat. No. 4,859,473, U.S. Pat. No. 4,410,667, and U.S. Pat. No. 4,410,668.
European patent application 0 561 152 discloses a coating composition comprising a polyester polyol and, optionally, a polyurethane polyol. This polyurethane polyol is only exemplified as a reaction product of a polyamine and a carbonate having a hydroxyl number of 400 to 800. The polyurethane polyol acts as a reactive diluent.
Furthermore, publications have been located disclosing coating compositions comprising polyurethane polyols but not polyester polyols: EP-A-0 530 806, EP-A-0 409 300, EP-A-0 409 301, and EP-A-0 455 291. Finally, U.S. Pat. No. 4,859,791 and GB 1,037,941 describe coating compositions comprising polyester polyols but not in combination with a polyurethane polyol.